<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    #box {
      padding: 20px;
      background: rgb(230, 229, 221);
    }
  </style>
</head>
<body>
  <div id="box"></div>
  <button id="btn1">执行计算任务</button>
  <button id="btn2">更改背景颜色</button>
  <script>
    var box = document.getElementById("box")
    var btn1 = document.getElementById("btn1")
    var btn2 = document.getElementById("btn2")

    var number = 99999
    var value = 0

    function calc (deadline) {
      while (number > 0 && deadline.timeRemaining() > 1) {
        number --
        value = Math.random() < 0.5 ? Math.random() : Math.random() // 产生昂贵的计算
        console.log(value)
      }
      requestIdleCallback(calc)
    }
    btn1.onclick = function () {
      requestIdleCallback(calc)
    }

    btn2.onclick = function () {
      box.style.background = 'green'
    }
  </script>
</body>
</html>